Currently vehicle air-bags are generally formed from woven nylon fabric. Nylon (along with other polyamides) has been used in the manufacture of air-bags (and in a large number of other applications) for many years, and its properties are well-understood.
In recent years, however, there has been a drive to use materials other than nylon, in particular for reasons of cost and environmental impact. It is preferred, if possible, to use polyester (or similar materials) in the manufacture of air-bags. This switching of materials does, however, present challenges, as it is often not straightforward to manufacture air-bags having advantageous properties to match those made using nylon.